Toast sesame seeds in a small dry skillet over medium-low heat, stirring constantly, until lightly browned and fragrant, 2 to 3 minutes. Transfer to a bowl to cool.

Mix 1/4 cup water, vinegar and cornstarch in a small bowl. Add black bean sauce and stir until smooth.

Heat oil in a large nonstick skillet or stir-fry pan over medium-high heat. Add garlic and stir-fry until fragrant, about 30 seconds. Add broccoli and stir to coat. Add the remaining 1/4 cup water; cover and steam just until the broccoli is tender-crisp, 1 to 3 minutes. Push broccoli to the sides and pour the sauce mixture in the center. Stir until the sauce begins to thicken, about 1 minute. Stir in the broccoli to coat. Serve immediately, sprinkled with the sesame seeds.